High speed and high density signal transmission among a plurality of electronic devices or among a plurality of circuit boards with an ordinary electric circuit is now encountering a limitation in increase of the speed and density due to mutual interference and attenuation of the electrical signals as barriers.
In order to overcome such limitation in increase of the speed and density, a technique of connecting among electronic devices and among circuit boards by means of light, so-called optical interconnection, is proposed. The combination of electrical wiring and optical wiring has been variously studied for the practical use of an optical interconnection.
An optical waveguide, which has a higher flexibility of wiring and has a capability of more highly densifying wiring compared with an optical fiber, is desired to be used as an optical transmission channel connecting among electronic devices and among circuit boards. In particular, an optical waveguide formed by using a polymer material with excellent processability and economic efficiency and an optical fiber connector provided with this optical waveguide show promise (for example, see Patent Document 1).
As shown in FIG. 12, an optical waveguide 300 generally is laminated to a flexible printed circuit board 340 on which an adhesive layer 342 is formed, and has a lower clad layer 322, a core pattern 324 formed on the lower clad layer 322, the core pattern forming an optical path through which a light signal transmits, and a mirror 330 formed on the inclined surface on the optical path of the core pattern 324 to change the direction of a light signal.
The mirror 330 is formed by deposition, sputtering, plating, and the like. For example, deposition or sputtering is conducted by using a metal mask with an opening corresponding to the inclined surface. Alternatively, the inclined surface is opened by using a resist, deposition, sputtering, or plating is conducted, and then the resist is removed. A mirror protection clad 328 is laminated to the opening in order to protect the mirror 330.
The mirror 330 changes the direction of a light signal so that a light signal transmitting through the optical path exits from the clad layer 322 and so that a light signal entering from the lower clad layer 322 transmits through the optical path.
Generally, the inclined surface on the optical path is formed by cutting with a dicing saw or the like after a lower clad layer, a core pattern, and an upper clad layer are formed. However, when the inclined surface is formed with a dicing saw, the substantial refractive index difference cannot be secured because the boundary between the core and the clad is continuous. Thus, the visibility of the mirror formed on the core may be reduced. Moreover, the reduced thickness of the optical waveguide at the cut portion has a risk of crashing or breaking the product in the following production process of forming specular metal.
Furthermore, an optical waveguide having a void-shaped air reflective mirror changing the optical path by using the refractive index difference between the inclined surface and the air layer has the above-mentioned risk not only in the production process but also after the production process ends.
Another optical waveguide with mirror improves the strength of the position at which the mirror is formed by installing a reinforcing plate on the inclined surface so that the inclined surface of the above-mentioned air reflective mirror is not embedded (for example, see Patent Document 2).
However, in such an optical waveguide with a reinforcing plate, the air layer formed between the mirror for changing an optical path and the reinforcing plate connects with the space outside of the housing of the optical waveguide through at least two positions. Thus, when the optical waveguide is fixed to a connector or another housing by using a liquid adhesive or when the optical waveguide is washed with water for clearing the substrate, capillarity causes liquid to infiltrate into the air layer formed between the mirror for changing the optical path and the reinforcing plate. This causes the mirror for changing the optical path not to reflect light well.